The invention relates to a circuit arrangement for at least one sensor element of at least one touch contact switch, particularly operating on a capacitive basis. A control signal having a specific operating frequency is applied to the sensor element and is modified as a function of the operating state of the sensor element. The sensor element can e.g. be operated by the approach or application of a finger, which is intended to represent a switching signal. This switching signal is detected and evaluated by the circuit arrangement and is transmitted to further switching devices.
A circuit arrangement of this type is e.g. known from DE 19 706 167 or U.S. Pat. No. 5,270,710. The circuit arrangements are either relatively complicated or have inadequate operating characteristics.
The object of the invention is to provide a circuit arrangement of the aforementioned type, which has a simple construction and can be constructed with a relatively small number of components used in a standard manner and which minimizes the influence of faults.
This problem is solved by a circuit arrangement having the features of claim 1. Advantageous developments of the invention form the subject matter of subclaims and are also explained hereinafter.
According to the invention the circuit arrangement has a signal source, which supplies the control signal, which is generally an alternating current signal. It also contains a transistor, to whose base is applied the control signal by means of a filter and/or a voltage divider. The filter is preferably constituted by a pass filter. By means of a resistor an at least minimum base current is produced at the transistor. The transistor collector is connected to earth or ground by means of a capacitor. The output signal of the circuit arrangement can be tapped at the transistor collector for further processing. Between the transistor base and ground is connected the sensor element, which is a capacitive sensor element. Apart from the filtering action, the filter can also be used for producing a signal shift.
A particularly preferred sensor element is e.g. described in U.S. Pat. No. 5,973,417 Goetz et. al, whose content is by express reference made into part of the content of the present description. It can be constructed in the manner of a unitary body from conductive foam. By the application of a finger of a user to this sensor element a capacitive coupling of the user to ground is produced and the sensor element, together with the user, can be seen as a capacitor connected to ground. The use of other sensor elements is obviously also possible.
The pass filter is advantageously a high-pass filter and can be constructed from a resistor and a capacitor.
The voltage divider can be of a capacitive nature. It is advantageously constructed in such a way that a capacitor of the voltage divider is a component of the aforementioned pass filter. Resistors can also form part of the voltage divider. This leads to a reduction in the number of components.
Preferably a resistor is provided and by means thereof at least the capacitor between the transistor collector and ground can be discharged. This discharge process occurs in a phase in which the control signal has a zero or lower potential. Advantageously also the capacitor of the capacitive voltage divider and/or a capacitor of the pass filter can be discharged, in particular also across said resistor.
The output signal which can be tapped at the transistor collector is supplied for further processing e.g. to a microcontroller, which determines whether an operation or a desired switching initiation has taken place. This can in particular be established by a disruption of the control voltage if during operation a further capacitor is connected to ground. Through the use of the present invention it is possible to obtain such a large signal swing of the circuit that an evaluation of the output signal can be carried out without an amplifier. In particularly preferred manner a unipolar square wave signal constitutes the control signal.
These and further features can be gathered from the claims, description and drawings and the individual features, either individually or in the form of subcombinations, can be implemented in an embodiment of the invention and in other fields and can represent advantageous, independently protectable constructions for which protection is hereby claimed. The subdivision of the application into individual sections and the subheadings in no way restrict the general validity of the statements made thereunder.